I was able to get them to spawn in a custom forest biome. However, even making their spawn settings better than cows and removing other passives, seemed like I had to go several rounds of tweaking and resetting the world for other reasons (adjusting terrain, objects, etc) before I noticed them. I'm not sure what this indicates without more focused testing.
Paperclip build 1139 (1.12)
Terrain Control build 179 (2.8.2 snapshot)